Very many processes and apparatuses for producing an islands-in-sea type composite fiber have heretofore been proposed. However, even when the number of island parts can be increased, there is the problem that the mass proportion (island proportion) of the island parts to the sea part is hard to increase. That is, when an attempt is made to increase the island proportion, the following problems arise: the relationship between the sea part and the island parts is reversed, and the problem that the polymer used for forming the island parts is brought into a continuous state to form the sea part; and the problem that even though the number of island parts can be increased, the area of each extrusion orifice of the spinneret becomes very large. Moreover, in such cases, the sites and number of the island parts are hard to adjust, and various problems arise to such a degree that a nonuniform composite fiber cannot be obtained.
For example, a process for producing an islands-in sea type composite fiber having very many islands is disclosed in Patent Reference 1 (JP-58-12367-B). This process comprises the steps of forming a plurality of island-in-sea type composite streams in an up stream part of a spinning system, gathering the composite streams in each of a plurality of the primary funnel-like portions, mutually gathering the primary gathered composite streams at a secondary funnel-like portion arranged in the downstream part of the spinning system, and extruding the resultant secondary gathered composite stream through an extrusion orifice. The process surely increases the number of islands. However, the extrusion orifices of the spinneret are complicated and costly, and the operationability in the production steps is difficult. Moreover, in order to prepare fine fibers wherein the number of the island parts is 200 or more, and the thickness of the island parts is 0.0095 dtex or less, the amount of the sea part must be increased. The mass ratio of the sea part to the island parts must therefore be 1:1 or more. As a result, the problem that the amount of a sea part polymer to be dissolved and wasted is still large, remains.
On the other hand, in Patent Reference 2 (JP-60-28922-B), a process for producing a fiber composed of an aggregate of fine and short polymer fibers is disclosed. This process comprises the steps of forming an islands-in-sea type mixed spun fiber from mixed composite polymers prepared by using a static mixer, and removing the sea part. However, because the island phases are formed by blending, the uniformity is inadequate. Moreover, the fiber is an aggregate fiber formed from fine fibrils each having a finite length in the longitudinal axis direction of the fiber. As a result, the fiber has the problem that the strength is low.
[Patent Reference 1] Japanese Examined Patent Publication (Kokoku) No. 58-12367
[Patent Reference 2] Japanese Examined Patent Publication (Kokoku) No. 60-28922